Subnet
Masks - Implementing subnewtorks (commonly referred to
as subnets in the field) helps to control network traffic.
Every node on the same physical Ethernet network sees all
the packets of data sent out on the network. This commonly has
the result of multiple collisions causing network
performance to be slow. Routers or gateways are used to
separate networks into subnets. Subnet masks on each of the
nodes allow the nodes on the same subnetwork to continue to
communicate with one another and with the routers or gateways
they use to send their messages.

Subnet masks allow you to identify the network ID and the
host (node) ID of an IP address.

we can see that rather than having the single subnet and
65534 Hosts per Subnet allowed under the default subnet mask,
we are able to have up to 16 subnets with up to 4094 Hosts
per Subnet by using a Subnet Mask of 255.255.240.000.